This invention relates generally to electrophotographic printing, and more particularly concerns an optical system employed therein for forming a half-tone light image of an original document being reproduced.
Conventional screening methods employed in electrophotographic printing machines produce the effect of tone graduation by means of dot size variations. In the highlight regions, the dots are small and increase in size through the intermediate shades until they merge together in the shadow regions. At the extremes, there is complete whiteness at the highlight end of the tone scale and nearly solid black at the shadow end. This type of tone structure can be reproduced in an electrophotographic printing machine.
Many techniques have been developed to improve half-tone reproductions. In graphic arts, moire patterns are minimized by changing the screen orientation between successive single color half-tone patterns. With the advent of colored electrophotographic printing, screening techniques have been employed to improve copy quality. Multi-colored electrophotographic printing is substantially identical to black and white printing. The process of black and white electrophotographic printing is described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942. In multi-color electrophotographic printing, the light image is filtered producing successive single color light images of the original document. These colored light images expose a charged photoconductive surface to create successive single color electrostatic latent images thereon. Each single color electrostatic latent image is developed with toner particles complementary in color to the filtered light image. The toner powder images are transferred from the electrostatic latent images to a sheet of support material, in superimposed registration with one another. This multi-colored powder image is permanently affixed to the sheet of support material forming a color copy thereon.
Half-tone screens are used in multi-colored electrophotographic printing system to enhance the copy being reproduced thereby. The screen is interposed into the optical light path and successive single color light images are transmitted therethrough onto the charged photoconductive surface to form an image of dots. In multi-color lithography and gravure reproductions, the screen may be rotated to discrete orientations for each single color image. By way of example, U.S. Pat. No. 3,158,479 issued to Pluess in 1964 describes such a concept.
It is a primary object of the present invention to improve the optical system of a multi-color electrophotographic printing machine by substantially optimizing the half-tone single color light images and the contrast thereof.
Another object of the present invention is to provide a convenient and rapid system for producing the proper discrete orientations for each single color image in a full color half-tone reproduction.